"Today is a day that historians will mark as a milestone for equal rights," she said to a hailing crowd at the state Capitol in Olympia.

The House passed the bill with a 55-43 vote on Feb. 8, one week after the Senate approved it. The gay marriage law is slated to take effect June 7.

Opponents, however, are determined to overturn the measure by collecting enough signatures to send it to the ballot box. If they collect more than 120,000 names by June 6, as expected, the law will be put on hold until the November election.

The state's Roman Catholic bishops have led the opposition to the same-sex marriage law, saying only traditional marriage upholds "the stability of family life in which a man and a woman conceive and nurture new life" as the basis for "a sound and functioning society."

"If asked, the voters of Washington state will say yes to marriage equality. ... I believe Washingtonians know it's time to give our gay and lesbian couples the right to a marriage license," said Gregoire, a Catholic.

A recent poll by the University of Washington Center for Survey Research found that if the issue does go to the voters, most would support it. Of those surveyed, 55 percent said they would uphold the same-sex marriage law if it were challenged by referendum and 38 percent said they would vote against it.

Washington has had a domestic partnership law since 2007, and in 2009 an "everything but marriage" expansion that was upheld by voters.

Also on Monday, the New Jersey state Senate passed gay marriage legislation. Gregoire said she will make an appeal to the state's Republican governor, Chris Christie, who has said he will veto the bill.